Why do we sleep? Companion studies in mice by NIMH grantees add to evidence that sleep may be the price we pay for the ability to learn.

The results suggest that sleep streamlines neural connections, or synapses, making them more efficient. It likely restores homeostatic balance to energy resources spent by learning during wakefulness, say the researchers.

This is accomplished by the downsizing and weakening of unneeded connections –a selective forgetting, so to speak. A team led by Giulio Tononi, M.D., Ph.D., and Chiara Cirelli, M.D., Ph.D. , at the University of Wisconsin found that 80 percent of cortex synapses sampled shrunk by nearly 20% during sleep.

Another team led by Richard Huganir, Ph.D., of Johns Hopkins University found that key cell surface receptor proteins in those synapses similarly plummet during sleep – and ID’d the underlying molecular mechanisms.

All this makes room for new learning, according to the synaptic homeostasis hypothesis proposed by the researchers.

Neural connections form at the tips of brain cell’s branch-like extensions. Such synapses grew during wakefulness and shrank during sleep, likely refreshing learning ability.